1. Technical Field of the Invention
The present invention relates to a key switch which is incorporated in a keyboard as an I/O interface.
2. Description of the Related Art
The key switch to be incorporated in the keyboard is such a device that its size may not change with a view to conforming to a shape of a man's finger, for convenience of operating with the finger. Moreover, the key switch is the device that such features as satisfying the man's feeling of operation may be particularly required.
It is required for the key switch to ensure smooth up and down movements free from a backlash or inclination of a key top, and strokes capable of obtaining, during a pressing stroke, feeling of buckling or tactile feeling for the purpose of feeding back ON or OFF shift of a contact to the finger, feeling of depressing operation, and feeling of shifting the contact.
Particularly, the smooth up and down movements of the key top free from a backlash or inclination have been required. In the related art, as a guiding and supporting mechanism for that purpose, there have been often employed two link members which are pivotally connected to each other at substantially center parts thereof to be formed into a substantially X-shape. This guiding and supporting mechanism has been keenly required to reduce its height, in a notebook type rather than in a desktop type. Since the notebook type needs to be folded, it has been a potential problem to reduce an overall height of the guiding and supporting mechanism including the key top.
FIG. 6 is a sectional view of a related art key switch (Refer to JP-A-2003-297177, for example).
In FIG. 6, a holder 102 in a substantially π-shape is engaged with a back face of a key top 101 by means of locking hooks 103 and 104. Locking pins provided at ends of a guiding and supporting mechanism 105 are arranged between the key top 101 and the holder 102. When the key top 101 is depressed or released, the guiding and supporting mechanism 105 will be extended in a substantially X-shape or folded telescopically to be superposed, changing its height. Especially when the guiding and supporting mechanism 105 has been folded and superposed, the height remains as it is, which will be a serious problem.
The guiding and supporting mechanism 105 includes link members 108, 109 which are assembled into a substantially X-shape by means of a shaft 110 in a columnar shape and a bearing part 111 in a shape of a columnar groove for receiving the shaft. The link members 108, 109 respectively have connecting portions. One of the connecting portions 106 is held between the back face of the key top 101 and the holder 102 so as to slide freely, and the other connecting portion 107 is clamped between the back face of the key top 101 and the holder 102.
The guiding and supporting mechanism 105 is required to have strength because of necessity of receiving an operating force of the finger, and so, the shaft 110 and the bearing part 111 which are affected by the force have been unable to be made small in diameter from a viewpoint of strength. For this reason, when the key top 101 has been depressed to the lowest, and the guiding and supporting mechanism 105 has been folded into a rectilinear shape, it has been impossible to make the height of the guiding and supporting mechanism 105 smaller than the height of the bearing part 111 which bears the shaft 110 passed therethrough.
Under the circumstances, there has been proposed such a structure that instead of rotatably supporting the link members 108, 109 by the related art bearing mechanism including a circular (a perfect circle) shaft 110 and the bearing part 111 in a shape of a circular groove, the link members are rotatably supported by means of two heart-shaped projections which have smaller thickness than the circular shaft (Refer to JP-A-6-60769, for example).
FIGS. 7A to 7C show a structure of the guiding and supporting mechanism in a substantially X-shape which rotatably supports the link members by means of the two heart-shaped projections in the related art. FIG. 7A shows an outer link member in a U-shape provided with outer leg pieces, FIG. 7B shows an inner link member in a U-shape provided with inner leg pieces, and FIG. 7C shows the structure of the guiding and supporting mechanism in which both the leg pieces are assembled into the substantially X-shape.
The guiding and supporting mechanism 120 includes the outer link member 122 in a U-shape provided with the outer leg pieces 121a, 121b, and the inner link member 123 in a U-shape provided with the inner leg pieces 123a, 123b, which are assembled into the substantially X-shape.
A pair of the outer leg pieces 121a, 121b are provided with heart-shaped projections 125, 126 at different levels on their inner faces opposed to each other. The outer leg pieces 121a and 121b are connected by a connecting rod 129 which is provided at their one ends.
In the same manner, a pair of the inner leg pieces 123a, 123b are provided with heart-shaped projections 127, 128 at different levels on their outer faces opposed to each other. The inner leg pieces 123a and 123b are connected by a connecting rod 130 which is provided at their one ends.
These two link members are arranged in the substantially X-shape as shown in FIG. 7C, and the heart-shaped projections 125 to 128 of all the leg pieces 121a, 121b, 123a, 123b are brought into contact as shown in the drawing thereby to support the X-shape so as not to be collapsed.
Although the above described guiding and supporting mechanism includes the link members assembled into the substantially X-shape, there has been proposed another type of guiding and supporting mechanism in which the shafts are not employed. Instead of assembling the link members into the substantially X-shape, the link members having the same shape are arranged separately, and rotatably engaged with each other at their one ends so that the two link members may be inclined at a same inclination angle when a key top is depressed (Refer to JP-A-8-124456, for example).
In the typical guiding and supporting mechanisms in the above described related art too, a problem of reducing the height of the guiding and supporting mechanism has become more and more important. In the related art guiding and supporting mechanism 105, the height of the link members can be reduced when the guiding and supporting mechanism 105 has been folded, because they are rotatably connected by means of a shaft, specifically, the shaft 110 and the bearing part 111 in a circular shape. However, the height corresponding to the diameters of the shaft 110 and the bearing part 111 cannot be reduced, but remains as it is. Moreover, the holder 102 has been employed so as to be opposed to the key top 101, due to necessity of providing a mechanism for fixing or slidably guiding one of the connecting portions of the link members on the key top. This holder 102, however, has been an obstacle for reducing the overall height.
On the other hand, in the guiding and supporting mechanism in which the link members are rotatably supported by the heart-shaped projections 125 to 128 which have smaller thickness than the shafts in a circular shape, positions of a pair of the heart-shaped projections provided on the respective link members are staggered in a vertical direction. Specifically, the heart-shaped projection 126 of the outer leg piece 121b is at a higher position than the heart-shaped projection 128 of the inner leg piece 123b, and the heart-shaped projection 125 of the outer leg piece 121a is at a lower position than the heart-shaped projection 127 of the inner leg piece 123a. 
In the guiding and supporting mechanism 120 as shown in FIG. 7C, when the outer link member 122 has been depressed, the heart-shaped projection 126 presses the heart-shaped projection 128 downward, but the heart-shaped projection 125 moves apart from the heart-shaped projection 127 without pressing it downward. As the results, in the inner link member 124, only the inner leg piece 123b will be depressed, while the inner leg piece 123a will not be directly depressed, but simply follow the movement by way of the connecting rod 130. Hence, a twisting force around a fixed shaft 131 will be exerted, and a prying phenomenon will occur between the inner leg piece 123a of the inner link member 124 and the outer leg piece 121a of the outer link member 122, which will hinder smooth up and down movements.
Moreover, when the finger is detached from the key top in a depressed state, the guiding and supporting mechanism 120 will be returned to an initial state by means of a return spring (not shown). On this occasion, the heart-shaped projection 125 presses the heart-shaped projection 127 upward, but the heart-shaped projection 126 will not press the heart-shaped projection 128, but will move apart from the heart-shaped projection 128.
Further, in the guiding and supporting mechanism in which the two link members are adapted to be inclined at the same inclination angle, the link members draw a circle and the key top also will draw a circle. For this reason, the guiding and supporting mechanism becomes inconvenient for use, because an unnecessary play corresponding to the circle is required in arranging the key tops, and movement of the key top will be misaligned with movement of the finger.